pair notification method + dto

LightlessSyncAPI/Dto/User/UserPairNotificationDto.cs

@@ -0,0 +1,10 @@
+using MessagePack;
+
+namespace LightlessSync.API.Dto.User;
+
+[MessagePackObject(keyAsPropertyName: true)]
+public sealed class UserPairNotificationDto
+{
+    public required string myHashedCid { get; init; }
+    public required string message { get; init; }
+}